The heroic sewer rats of the Somme: Truly humbling stories of the horrors of tunnel warfare 80ft underground as seen in BBC1's Birdsong

The new BBC TV adaptation of Birdsong - starring Clemence Poesy as Isabelle Azaire and Eddie Redmayne as Stephen Wraysford - depicts the horrors of tunnel warfare in World War I

The weary soldier in the World War I trenches of northern France left his own poignant memento for posterity. ‘If in this place you are detained,’ he scratched on a wall, a labour of love that must have taken him days if not weeks, ‘don’t look around you all in vain/ But cast your net and ye shall find/ That every cloud is silver-lined.’

What was remarkable about such a hope-filled message was that the unknown man who left it spent his military service deprived of sky and clouds, silver-lined or otherwise. He fought his war underground, in stifling heat and near-darkness pierced only by candlelight.

He was one of thousands of sappers — privates in the Royal Engineers — who laboured below the surface of no man’s land, digging tunnels towards the enemy’s frontline that could be filled with explosives and then detonated to blow the German trenches to high heaven.

Sappers were mainly former coal miners drafted in from the pits, but their numbers also included those who had been digging deep city sewers and the tunnels of the London Underground. Now they were tunnelling as an act of war.

As if the dangers of being buried alive or drowned by floodwater which ran into the battlefield tunnels were not enough, these ‘moles’ and ‘sewer rats’, as they were known, were also grimly aware that their German equivalents were doing exactly the same thing from the other direction. At any moment, they might be just a few feet away.

Eddie Redmayne as Stephen Wraysford depicting life as a soldier in World War I in the BBC TV adaptation of Birdsong

British troops negotiating a trench as they go forward in support of an attack on the village of Morval during the Battle of the Somme. The date of July 1, 1916, is remembered as the bloodiest day in the history of the British Army

Sappers at work on the Western Front in World War I. The struggle in the tunnels was one of the least-known battlefronts of the war. It had horrors all of its own and was fought by a very special type of man

As the British sappers squatted in the dark and heaved their picks and spades through the cloying earth, snatching their breath from the hot, fetid air and struggling with claustrophobia, they had one ear open for the fall of soil that might threaten a cave-in or the hiss from a pierced pocket of gas that could asphyxiate them — and the other for the tip-tap of enemy spades.

To disguise their own presence, they muffled their picks and padded through the tunnels in their socks.

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Sometimes — as in Sunday’s first part of the BBC TV adaptation of Sebastian Faulks’s novel Birdsong — a tunnel would be breached by the enemy, and British and German miners would suddenly find themselves face to face.

Then, rifles would bark in the dark, bullets fly, bayonets thrust, and the intense man-to-man fighting on the crater-scarred surface of the Somme would break out in those subterranean hell holes.

After a bombardment, sappers - with gas masks on - lay a telephone cable in the final year of the Great War. And (right) soldiers in the Royal Irish Rifles during a pause in fighting during the Battle of Somme in the summer of 1916

Considering their options. A scene from TV dramatisation of Birdsong. Pictured are Lt Stephen Wraysford (played by Eddie Redmayne, centre) and Byrne (Matthew Aubrey, left). The unit led by Lt Wraysford was tasked with supporting the miners from the surface by pumping air down to them and carrying away ton after ton of soil

The Germans started the practice of tunnelling, digging the first one in December 1914 just 50 miles from Calais and exploding ten smallish mines under the trenches of a British brigade.

This led to the urgent formation of British specialist tunnelling companies, formed of experienced coal miners from pits all over the country. They were soon at work, sinking shafts straight down through the sandy topsoil and then as much as 80ft into the firm Flanders clay. From the main shaft, tunnels headed off towards the enemy, in round-the-clock operations that never stopped, not even for Christmas.

By mid-1916, a small army of 25,000 tunnellers were burrowing away, supported by twice that number of infantrymen, who — like the unit led by Lieutenant Stephen Wraysford, the hero of Birdsong — were tasked with supporting the miners from the surface by pumping air down to them (and water out) and carrying away ton after ton of soil.

Only when he is ordered, against his every instinct, to go underground with the miners does Wraysford come to appreciate the horror of the tunnellers’ lives, worse even than the mud and squalor of the trenches.

The struggle in the tunnels was one of the least-known battlefronts of the war. It had horrors all of its own and was fought by a very special type of man.

Typical of the breed was William Hackett, a South Yorkshire miner who before the war specialised in making and repairing roads and railways underground. He tried to join up as a frontline soldier but, at 41, was rejected as too old.

Instead he joined the Royal Engineers and, despite a dicky heart, was soon underground again. It was dangerous work, requiring rare courage, as Hackett showed.

In the early hours of a June morning in 1916, he and four others were 35ft down pushing towards the enemy lines in a timbered gallery 3ft wide and 4ft high. Suddenly a German mine exploded.

On the surface, the ground heaved upwards, spewing earth into the air, then gaped open in a massive crater. There was devastation in the British frontline, which the Germans exploited by attacking. Many men died on both sides before the enemy were repulsed.

Life in the trenches: Royal Fusiliers fixing their bayonets in a trench on July 1, 1916

Rear of the Thiepval Monument in France which honours 72,000 British and South African servicemen who fell at the Battle of the Somme in World War One

But below ground the devastation was
terrible, too. The mine had collapsed 25ft of the tunnel, cutting off
Hackett and his mates from escape.

For two days, rescuers dug down, despite German shelling and mortaring of the top of the shaft, scraping out an escape hole through which eventually three of the men were able to crawl to safety.

But, though he could have got away, Hackett refused to leave the last man, a Welsh miner who was badly injured.

The roof collapsed and both men died — two of the 3,000 miners who lost their lives under the Western Front rather than on it. Many of their bodies were never recovered and still lie entombed in those tunnels. Hackett was awarded a posthumous Victoria Cross, the only tunneller to receive this ultimate award for gallantry.

Underground mine: Men working a mine-drill in a gallery, making a bore-hole for a 'camoflaut' or secondary chamber, intended to cause the enemy work to fall in

While on the surface, the battlefield was a stalemate, with little movement on the opposing frontlines 150 yards apart, in the spreading labyrinth of tunnels underground, complex manoeuvres and feints, attacks and defences were taking place. If one side got wind of an enemy tunnel, they would begin one of their own to try to intercept it, in a deadly game of what has been termed ‘blindfold cat and mouse’.

Often the enemy’s movements were obvious. An officer recalled the Germans at work just 16 paces away. ‘I could hear them spit.’ Listening became a specialist job in itself, and trained eavesdroppers lay silently in the dark, trying to pinpoint the sounds of enemy picks and shovels.

The naked ear was supplemented by techniques developed by the water companies back home to detect shifts in the soil and underground leaks. Later on in the war, high-tech (for the times) devices were introduced, with microphones, stethoscopes and remote sensors which detect movement. From the triangulation of compass bearings, enemy tunnels could be tracked and destroyed.

As a result, the vast majority of underground explosions were not grand ones carving huge chunks in the opposite frontline, but smaller ones targeting enemy tunnels. Who had the nerve to light the fuses first was often crucial. After German miners were detected edging towards the British frontline in October 1915, an 80ft shaft was sunk and galleries dug in the direction of the German tunnel.

Captain Henry Hance spent six hours in the furthest chamber, eavesdropping on the Germans, and concluded they were 15 yards away. He ordered the chamber packed with 6,000lb of explosives, a laborious and time-consuming process at that depth.

But the Germans blew theirs first. This not only detonated the British explosives and wrecked the tunnels but released a massive quantity of carbon monoxide gas, which killed eight British miners.

No wonder then that, on the surface, secrecy was paramount and great efforts made to conceal any sign of a tunnel being dug.

Since tunnels often took months of intense work by hundreds of men, letting the cat out of the bag was a disaster.

The classic novel Birdsong by Sebastian Faulks, left, depicts a soldier's life during the battle of the Somme, in fields in the Valley of the Somme, near, Mons, Nord-Picardie, which is now covered with red poppies, right

But, if the secret held, there were notable successes. At the Battle of Messines in June 1917, 600 tons of explosives were placed in 21 tunnels running underneath a German-held ridge. They ran for 4½ miles and had taken a year-and-a-half to dig. Originally there were 22, but the Germans discovered one.

At 3.10 in the morning, they were blown with an explosion that could be heard in London. A vast pillar of fire lit up the sky and the entire crest of the Messines ridge was blown off.

Ten thousand German soldiers were killed in an instant. The British line advanced to take its objective, a rare advance in the stalemate on the Western Front.

Two mines did not, for some reason, detonate, and to the chagrin of locals, the British mislaid the tunnels’ precise location.

One finally went off in 1955, killing a cow. The other is undiscovered, its deadly blast against a long-gone enemy still waiting to go off.